This invention relates to electronic circuitry for generating ultrasonic signals and for sensing reflected ultrasonic echo signals; and more specifically to such electronic circuitry for use as ultrasonic tension control in manual or automated fastener tightening.
Ultrasonic signal processing has been used in the past to detect flaws in metal objects, such as fasteners, and to measure the elongation of a fastener during or after tightening. The procedure adopted has been to transmit electronically generated ultrasonic pulses down the length of a fastener and then to measure the time from pulse to echo, i.e. the reflected ultrasonic signal. This is the "time of flight" of the echo. The time of flight of the echo measures the distance to a fault, an inclusion or a fracture in a faulty fastener, and the length of a good fastener. However, the length of the fastener changes as tension is applied to the fastener. Therefore, a change in time of flight occurs as a function of axial tension.
Meisterling, U.S. Pat. No. 4,760,740, shows an extensometer unit coupled to an ultrasonic transducer which in turn is mounted to the head of a fastener. The Meisterling extensometer unit contains signal generating, signal receiving and signal processing circuitry of a general nature. An example of an extensometer circuit is shown by McFaul et al., U.S. Pat. No. 3,759,090.
Jones, U.S. Pat. No. 4,413,518, shows a bolt elongation measurement apparatus and a method of measuring bolts. The circuitry utilizes a microprocessor-based digital system including a binary counter which counts pulses generated by a high frequency oscillator during the time interval between the entry into the bolt of a first pulse and exit from the bolt of a second pulse derived from the reflection return of ultrasonic energy from the opposite end of the bolt. The count is applied to a computer for calculation of bolt length or of bolt stretch due to mechanical stress. The calculation also incorporates data input thereinto and corresponding to material velocity, stress correction factor, measurement temperature, and thermal correction factor. A digital filtering algorithm ensures an accurate and stable measurement. The receiver of the apparatus overcomes the problem of spurious ultrasonic reflection characteristics of threaded bolts by means of a dual characteristic echo sensing circuit to deal with stress-induced pulse distortion and a gain contour circuit to deal with spurious echo pulses that are characteristic of threaded bolts.
Couchman, U.S. Pat. No. 4,295,377, shows ultrasonic signal generation and detection circuitry which includes logic and timing circuits for generating an echo signal detection "window" This circuitry and detection technique of Couchman is also shown in his U.S. Pat. No. 4,294,122. The detection window establishes a time period when any signal received is taken as the desired echo pulse The time lapse between detection windows is adjustable and a function of the repetition rate of the primary ultrasonic pulses which Couchman provides at 100 to 2000 pulses per second.
Moore, U.S. Pat. No. 4,014,208, shows an ultrasonic device for measuring dimensional changes in a structural member. The device includes circuitry to double pulse a transducer to transmit an acoustic pulse into the member at one end for reflection from its other end with a period between paired pulses selected to cause the second echo received of the first pulse to coincide with the first echo of the second pulse. A voltage controlled oscillator is employed with a digital counter to time the period between paired pulses, the interval between successive paired pulses, and the time of a predetermined number of pulse pairs. The latter timing is used to alternatively shift the frequency of the voltage controlled oscillator to cause the first echo of the second pulse to be offset in phase from the coincidence position it might have at the central frequency. Phase detection and integration of the echo pulse coincidence during alternately high and low frequency offsets produces a phase-sensitive feedback signal to the voltage controlled oscillator to drive its central frequency toward precise coincidence.
Kibblewhite, U.S. Pat. No. 4,846,001, shows a fastener with an ultrasonic transducer affixed thereto. Electronic source pulses are applied to the transducer and electrical echoes produced by reflected ultrasonic waves are sensed. Kibblewhite measures a change in mechanical stress in the fastener by measuring a change in bolt stretch as a function of change in time of flight of echo measurement. Three time of flight measurement schemes are discussed. These are (1) a direct timing technique, (2) an indirect timing technique, and (3) a double pulsing technique.
A direct timing technique involves the measurement of the time interval between a source pulse (drive pulse) and the received echo. An indirect timing technique involves timing from the first echo to the second echo of a particular source pulse. In a double pulsing technique, two source pulses are transmitted, one after another. The time interval between these two pulses is adjusted so that the second echo from the first of the two source pulses coincides with the first echo from the second of the two source pulses.
Kibblewhite also discusses various echo detection techniques which can reduce the delay time of waiting for echoes of the previous pulse to die down. These detection techniques include (a) a fundamental frequency detection technique, (b) an acoustic impedance detection technique, (c) a harmonic resonance frequency detection technique and (d) a phase detection technique.
While the above-cited devices and methods can provide reliable information about a fastener, they have limitations in use. These techniques rely on averaging techniques to achieve their accuracy Therefore, they are capable of either high accuracy or high measurement rate and are generally used for taking measurements before and after tightening.
What is desired is an intelligent drive/sense ultrasonic signal circuit for measuring time of flight of pulse-echo time with greater accuracy.
What is secondly desired is ultrasonic signal drive/sense circuitry which achieves both high accuracy and high measurement rate and hence is useful for the control of ultrasonic measured tension during tightening.
What is further desired is ultrasonic signal drive/sense circuitry which does not require an extended delay time between transmitted pulses.
What is also desired is such drive/sense circuitry which uses a window for detecting an echo pulse and which can automatically select an optimum echo detection threshold for detecting an echo pulse.
What is further desired is such drive/sense circuitry which can automatically adjust the time position window for echo detection.
What is additionally desired is such drive/sense circuitry which automatically adjusts pulse drive voltage to compensate for variations in ultrasonic transducer electrical and acoustic efficiency and in fastener geometry.
What is even further desired is such drive/sense circuitry which can operate in an interleaved pulsing mode where pulse time is chosen so that echoes from the previous pulses fall outside the time acceptance window of the current measurement.
What is additionally desired is such drive/sense circuitry which can be adjusted to measure time of flight from a pulse to its echo, or from a pulse to its successive echo (reflections) or from its echo to a successive echo of that echo.
What is further additionally desired is such drive/sense circuitry which can operate with pulse repetition rates up to 10 KHz.